Well as is, you should be able to run hpa without much modification. At most you might have to play with the springs. You will need a constant air adapter if you don't want to modify the body, and if you want to run a bottomline, you will need an universal mounting bracket (UMB) so you can attach an asa to the bottom of the grip.

The other option is to use a dummy 12gram. You need to drill a hole in the lower tube so you can screw an airline into the dummy 12gram. It is a little cheaper option but requires a hole drilled, which some people don't want to do. Titus on here sells them on his site: